The present invention relates to a current collector of positive electrode which is suitably used for a sodium-sulfur battery and the like, and a sodium-sulfur battery using the current collector of positive electrode.
A sodium-sulfur battery (hereinafter referred to as an “NaS battery”) is a sealed-type high-temperature secondary battery which is operated at the high temperatures ranging from 300 to 350° C., and has a structure in which a certain quantity of sodium, as a negative active material, and another certain quantity of sulfur, as a positive active material, are separately housed by use of a solid electrolyte (for example, β-alumina, β″-alumina, and the like) having a function to selectively allow sodium ions to permeate therethrough.
In a NaS battery 1 shown in FIG. 1, for example, a bottomed cylindrical solid electrolyte tube 13 is placed inside a positive electrode container 9 having a hollow cylindrical shape, with sodium 2 being used as a negative active material and sulfur 4 being used as a positive active material being housed separately, respectively inside and outside of the solid electrolyte tube 13.
The solid electrolyte tube 13 is connected to the positive electrode container 9 with an insulator ring 3 made of α-alumina and the like and a cylindrical fitting 5 is interposed between the tube and the electrode, and thus the positive electrode and the negative electrode sections are insulated from each other.
In the NaS battery 1, during discharge, the sodium 2, which is the negative active material, turns into sodium ions releasing electrons to an external circuit, and the sodium ions pass through the wall of the solid electrolyte tube 13 reaching the positive electrode section, where the sodium ions react with electrons supplied by the sulfur 4, which is the positive active material, and by the external circuit to produce sodium polysulfides, thereby generating a voltage on the order of 2 V.
On the other hand, during charge, by applying a voltage from an external circuit, the sodium polysulfides release electrons to the external circuit to produce sulfur and sodium ions, and the sodium ions pass through the wall of the solid electrolyte tube 13 reaching the negative electrode section, where the sodium ions are allowed to react with electrons supplied by the external circuit to be electrically neutralized, thereby the electrical energy being converted into chemical energy.
Generally, a current collector of a positive electrode 11 is interposed between the positive and negative electrodes, for the purpose of securing the electric conduction therebetween and reducing the internal resistance of the battery, since the sulfur 4 (positive active material) in a NaS battery is an insulating material. The current collector of the positive electrode 11 is a member made of a felt material based on carbon fibers or graphite fibers having conductivity, which member is impregnated with the sulfur 4, which, again, is used as the positive active material. The current collector is arranged so as to abut against both the interior circumferential surface of the positive electrode container 9 and the exterior circumferential surface of the solid electrolyte tube 13 so that the electric conduction between the positive and negative electrodes is secured and the internal resistance of the battery is reduced.
Furthermore, in the NaS battery 1, the surface of the current collector of the positive electrode 11, which surface abutting against the solid electrolyte tube 13, has a high-resistance layer made of such an insulating material as α-alumina, glasses, and the like. The high-resistance layer decreases the electric conduction in the neighborhood of the contact surface between the solid electrolyte tube 13 and the current collector of the positive electrode 11, thereby being capable of avoiding, during discharge, the situation where the electron donating-accepting reaction takes place exclusively in the neighborhood of the contact surface between the solid electrolyte tube 13 and the current collector of the positive electrode 11.
Accordingly, it is possible to prevent the degradation of the charge recovery characteristic (a phenomenon where the charge process is not completed because the charge reaction does not proceed even though the sulfur polysulfides remain available) caused by the increasing internal resistance of the battery due to the precipitating sulfur, an insulating material, in the above described neighborhood with developing the charge reaction.